<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> <html> <head> <meta http-equiv="Content-Type" content="text/html; charset=US-ASCII"> <title>Chapter 38. Boost.Unordered</title> <link rel="stylesheet" href="../../doc/src/boostbook.css" type="text/css"> <meta name="generator" content="DocBook XSL Stylesheets V1.78.1"> <link rel="home" href="index.html" title="The Boost C++ Libraries BoostBook Documentation Subset"> <link rel="up" href="libraries.html" title="Part I. The Boost C++ Libraries (BoostBook Subset)"> <link rel="prev" href="boost_units/TODO.html" title="TODO"> <link rel="next" href="unordered/buckets.html" title="The Data Structure"> </head> <body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"> <table cellpadding="2" width="100%"><tr> <td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../boost.png"></td> <td align="center"><a href="../../index.html">Home</a></td> <td align="center"><a href="../../libs/libraries.htm">Libraries</a></td> <td align="center"><a href="http://www.boost.org/users/people.html">People</a></td> <td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td> <td align="center"><a href="../../more/index.htm">More</a></td> </tr></table> <hr> <div class="spirit-nav"> <a accesskey="p" href="boost_units/TODO.html"><img src="../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="libraries.html"><img src="../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="index.html"><img src="../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="unordered/buckets.html"><img src="../../doc/src/images/next.png" alt="Next"></a> </div> <div class="chapter"> <div class="titlepage"><div> <div><h2 class="title"> <a name="unordered"></a>Chapter 38. Boost.Unordered</h2></div> <div><div class="author"><h3 class="author"> <span class="firstname">Daniel</span> <span class="surname">James</span> </h3></div></div> <div><p class="copyright">Copyright © 2003, 2004 Jeremy B. Maitin-Shepard</p></div> <div><p class="copyright">Copyright © 2005-2008 Daniel James</p></div> <div><div class="legalnotice"> <a name="unordered.legal"></a><p> Distributed under the Boost Software License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>) </p> </div></div> </div></div> <div class="toc"> <p><b>Table of Contents</b></p> <dl class="toc"> <dt><span class="section"><a href="unordered.html#unordered.intro">Introduction</a></span></dt> <dt><span class="section"><a href="unordered/buckets.html">The Data Structure</a></span></dt> <dt><span class="section"><a href="unordered/hash_equality.html">Equality Predicates and Hash Functions</a></span></dt> <dt><span class="section"><a href="unordered/comparison.html">Comparison with Associative Containers</a></span></dt> <dt><span class="section"><a href="unordered/compliance.html">C++11 Compliance</a></span></dt> <dd><dl> <dt><span class="section"><a href="unordered/compliance.html#unordered.compliance.move">Move emulation</a></span></dt> <dt><span class="section"><a href="unordered/compliance.html#unordered.compliance.allocator_compliance">Use of allocators</a></span></dt> <dt><span class="section"><a href="unordered/compliance.html#unordered.compliance.pairs">Pairs</a></span></dt> <dt><span class="section"><a href="unordered/compliance.html#unordered.compliance.misc">Miscellaneous</a></span></dt> </dl></dd> <dt><span class="section"><a href="unordered/rationale.html">Implementation Rationale</a></span></dt> <dt><span class="section"><a href="unordered/changes.html">Change Log</a></span></dt> <dt><span class="section"><a href="unordered/reference.html">Reference</a></span></dt> <dd><dl> <dt><span class="section"><a href="unordered/reference.html#header.boost.unordered_set_hpp">Header <boost/unordered_set.hpp></a></span></dt> <dt><span class="section"><a href="unordered/reference.html#header.boost.unordered_map_hpp">Header <boost/unordered_map.hpp></a></span></dt> </dl></dd> <dt><span class="section"><a href="unordered/bibliography.html">Bibliography</a></span></dt> </dl> </div> <div class="section"> <div class="titlepage"><div><div><h2 class="title" style="clear: both"> <a name="unordered.intro"></a><a class="link" href="unordered.html#unordered.intro" title="Introduction">Introduction</a> </h2></div></div></div> <p> For accessing data based on key lookup, the C++ standard library offers <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">set</span></code>, <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">map</span></code>, <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">multiset</span></code> and <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">multimap</span></code>. These are generally implemented using balanced binary trees so that lookup time has logarithmic complexity. That is generally okay, but in many cases a <a href="http://en.wikipedia.org/wiki/Hash_table" target="_top">hash table</a> can perform better, as accessing data has constant complexity, on average. The worst case complexity is linear, but that occurs rarely and with some care, can be avoided. </p> <p> Also, the existing containers require a 'less than' comparison object to order their elements. For some data types this is impossible to implement or isn't practical. In contrast, a hash table only needs an equality function and a hash function for the key. </p> <p> With this in mind, unordered associative containers were added to the C++ standard. This is an implementation of the containers described in C++11, with some <a class="link" href="unordered/compliance.html" title="C++11 Compliance">deviations from the standard</a> in order to work with non-C++11 compilers and libraries. </p> <p> <code class="computeroutput"><span class="identifier">unordered_set</span></code> and <code class="computeroutput"><span class="identifier">unordered_multiset</span></code> are defined in the header <<code class="computeroutput"><a class="link" href="unordered/reference.html#header.boost.unordered_set_hpp" title="Header <boost/unordered_set.hpp>">boost/unordered_set.hpp</a></code>> </p> <pre class="programlisting"><span class="keyword">namespace</span> <span class="identifier">boost</span> <span class="special">{</span> <span class="keyword">template</span> <span class="special"><</span> <span class="keyword">class</span> <span class="identifier">Key</span><span class="special">,</span> <span class="keyword">class</span> <span class="identifier">Hash</span> <span class="special">=</span> <code class="computeroutput"><a class="link" href="boost/hash.html" title="Struct template hash">boost::hash</a></code><span class="special"><</span><span class="identifier">Key</span><span class="special">>,</span> <span class="keyword">class</span> <span class="identifier">Pred</span> <span class="special">=</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">equal_to</span><span class="special"><</span><span class="identifier">Key</span><span class="special">>,</span> <span class="keyword">class</span> <span class="identifier">Alloc</span> <span class="special">=</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">allocator</span><span class="special"><</span><span class="identifier">Key</span><span class="special">></span> <span class="special">></span> <span class="keyword">class</span> <code class="computeroutput"><a class="link" href="boost/unordered_set.html" title="Class template unordered_set">unordered_set</a></code><span class="special">;</span> <span class="keyword">template</span><span class="special"><</span> <span class="keyword">class</span> <span class="identifier">Key</span><span class="special">,</span> <span class="keyword">class</span> <span class="identifier">Hash</span> <span class="special">=</span> <code class="computeroutput"><a class="link" href="boost/hash.html" title="Struct template hash">boost::hash</a></code><span class="special"><</span><span class="identifier">Key</span><span class="special">>,</span> <span class="keyword">class</span> <span class="identifier">Pred</span> <span class="special">=</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">equal_to</span><span class="special"><</span><span class="identifier">Key</span><span class="special">>,</span> <span class="keyword">class</span> <span class="identifier">Alloc</span> <span class="special">=</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">allocator</span><span class="special"><</span><span class="identifier">Key</span><span class="special">></span> <span class="special">></span> <span class="keyword">class</span> <code class="computeroutput"><a class="link" href="boost/unordered_multiset.html" title="Class template unordered_multiset">unordered_multiset</a></code><span class="special">;</span> <span class="special">}</span> </pre> <p> <code class="computeroutput"><span class="identifier">unordered_map</span></code> and <code class="computeroutput"><span class="identifier">unordered_multimap</span></code> are defined in the header <<code class="computeroutput"><a class="link" href="unordered/reference.html#header.boost.unordered_map_hpp" title="Header <boost/unordered_map.hpp>">boost/unordered_map.hpp</a></code>> </p> <pre class="programlisting"><span class="keyword">namespace</span> <span class="identifier">boost</span> <span class="special">{</span> <span class="keyword">template</span> <span class="special"><</span> <span class="keyword">class</span> <span class="identifier">Key</span><span class="special">,</span> <span class="keyword">class</span> <span class="identifier">Mapped</span><span class="special">,</span> <span class="keyword">class</span> <span class="identifier">Hash</span> <span class="special">=</span> <code class="computeroutput"><a class="link" href="boost/hash.html" title="Struct template hash">boost::hash</a></code><span class="special"><</span><span class="identifier">Key</span><span class="special">>,</span> <span class="keyword">class</span> <span class="identifier">Pred</span> <span class="special">=</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">equal_to</span><span class="special"><</span><span class="identifier">Key</span><span class="special">>,</span> <span class="keyword">class</span> <span class="identifier">Alloc</span> <span class="special">=</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">allocator</span><span class="special"><</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span><span class="special"><</span><span class="identifier">Key</span> <span class="keyword">const</span><span class="special">,</span> <span class="identifier">Mapped</span><span class="special">></span> <span class="special">></span> <span class="special">></span> <span class="keyword">class</span> <code class="computeroutput"><a class="link" href="boost/unordered_map.html" title="Class template unordered_map">unordered_map</a></code><span class="special">;</span> <span class="keyword">template</span><span class="special"><</span> <span class="keyword">class</span> <span class="identifier">Key</span><span class="special">,</span> <span class="keyword">class</span> <span class="identifier">Mapped</span><span class="special">,</span> <span class="keyword">class</span> <span class="identifier">Hash</span> <span class="special">=</span> <code class="computeroutput"><a class="link" href="boost/hash.html" title="Struct template hash">boost::hash</a></code><span class="special"><</span><span class="identifier">Key</span><span class="special">>,</span> <span class="keyword">class</span> <span class="identifier">Pred</span> <span class="special">=</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">equal_to</span><span class="special"><</span><span class="identifier">Key</span><span class="special">>,</span> <span class="keyword">class</span> <span class="identifier">Alloc</span> <span class="special">=</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">allocator</span><span class="special"><</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span><span class="special"><</span><span class="identifier">Key</span> <span class="keyword">const</span><span class="special">,</span> <span class="identifier">Mapped</span><span class="special">></span> <span class="special">></span> <span class="special">></span> <span class="keyword">class</span> <code class="computeroutput"><a class="link" href="boost/unordered_multimap.html" title="Class template unordered_multimap">unordered_multimap</a></code><span class="special">;</span> <span class="special">}</span> </pre> <p> When using Boost.TR1, these classes are included from <code class="computeroutput"><span class="special"><</span><span class="identifier">unordered_set</span><span class="special">></span></code> and <code class="computeroutput"><span class="special"><</span><span class="identifier">unordered_map</span><span class="special">></span></code>, with the classes added to the <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">tr1</span></code> namespace. </p> <p> The containers are used in a similar manner to the normal associative containers: </p> <p> </p> <pre class="programlisting"><span class="keyword">typedef</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">unordered_map</span><span class="special"><</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">,</span> <span class="keyword">int</span><span class="special">></span> <span class="identifier">map</span><span class="special">;</span> <span class="identifier">map</span> <span class="identifier">x</span><span class="special">;</span> <span class="identifier">x</span><span class="special">[</span><span class="string">"one"</span><span class="special">]</span> <span class="special">=</span> <span class="number">1</span><span class="special">;</span> <span class="identifier">x</span><span class="special">[</span><span class="string">"two"</span><span class="special">]</span> <span class="special">=</span> <span class="number">2</span><span class="special">;</span> <span class="identifier">x</span><span class="special">[</span><span class="string">"three"</span><span class="special">]</span> <span class="special">=</span> <span class="number">3</span><span class="special">;</span> <span class="identifier">assert</span><span class="special">(</span><span class="identifier">x</span><span class="special">.</span><span class="identifier">at</span><span class="special">(</span><span class="string">"one"</span><span class="special">)</span> <span class="special">==</span> <span class="number">1</span><span class="special">);</span> <span class="identifier">assert</span><span class="special">(</span><span class="identifier">x</span><span class="special">.</span><span class="identifier">find</span><span class="special">(</span><span class="string">"missing"</span><span class="special">)</span> <span class="special">==</span> <span class="identifier">x</span><span class="special">.</span><span class="identifier">end</span><span class="special">());</span> </pre> <p> </p> <p> But since the elements aren't ordered, the output of: </p> <p> </p> <pre class="programlisting"><span class="identifier">BOOST_FOREACH</span><span class="special">(</span><span class="identifier">map</span><span class="special">::</span><span class="identifier">value_type</span> <span class="identifier">i</span><span class="special">,</span> <span class="identifier">x</span><span class="special">)</span> <span class="special">{</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span><span class="special"><<</span><span class="identifier">i</span><span class="special">.</span><span class="identifier">first</span><span class="special"><<</span><span class="string">","</span><span class="special"><<</span><span class="identifier">i</span><span class="special">.</span><span class="identifier">second</span><span class="special"><<</span><span class="string">"\n"</span><span class="special">;</span> <span class="special">}</span> </pre> <p> </p> <p> can be in any order. For example, it might be: </p> <pre class="programlisting"><span class="identifier">two</span><span class="special">,</span><span class="number">2</span> <span class="identifier">one</span><span class="special">,</span><span class="number">1</span> <span class="identifier">three</span><span class="special">,</span><span class="number">3</span> </pre> <p> To store an object in an unordered associative container requires both an key equality function and a hash function. The default function objects in the standard containers support a few basic types including integer types, floating point types, pointer types, and the standard strings. Since Boost.Unordered uses <code class="computeroutput"><a class="link" href="boost/hash.html" title="Struct template hash">boost::hash</a></code> it also supports some other types, including standard containers. To use any types not supported by these methods you have to <a class="link" href="hash/custom.html" title="Extending boost::hash for a custom data type">extend Boost.Hash to support the type</a> or use your own custom equality predicates and hash functions. See the <a class="link" href="unordered/hash_equality.html" title="Equality Predicates and Hash Functions">Equality Predicates and Hash Functions</a> section for more details. </p> <p> There are other differences, which are listed in the <a class="link" href="unordered/comparison.html" title="Comparison with Associative Containers">Comparison with Associative Containers</a> section. </p> </div> </div> <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> <td align="left"><p><small>Last revised: December 14, 2015 at 09:19:45 GMT</small></p></td> <td align="right"><div class="copyright-footer"></div></td> </tr></table> <hr> <div class="spirit-nav"> <a accesskey="p" href="boost_units/TODO.html"><img src="../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="libraries.html"><img src="../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="index.html"><img src="../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="unordered/buckets.html"><img src="../../doc/src/images/next.png" alt="Next"></a> </div> </body> </html>